1. Field
One embodiment of the present invention relates to an apparatus for cancelling resonance in the outer-ear canals and a method of cancelling resonance in the outer-ear canals.
2. Description of the Related Art
When a person is listening to music through an earphone or a headphone, resonance may develop between the eardrum and the earphone or the headphone. In this case, the music sounds strange to the listener. Various systems have been developed, which cancel such resonance. (See, for example, Jpn. Pat. Appln. KOKAI Publication No. 2000-92589, paragraph 0047 and FIGS. 1 and 2; Jpn. Pat. Appln. KOKAI Publication No. 2002-209300, paragraph 0040 and FIG. 1; and Jpn. Pat. Appln. KOKAI Publication No. 9-187093, paragraph 0024 and FIG. 2).
Jpn. Pat. Appln. KOKAI Publication No. 2000-(hereinafter referred to as Publication 1) discloses a technique of finding the position of an acoustic image outside a listener's head. FIGS. 2(a) and 2(b) of Publication 1 illustrate the principle of finding the position of the acoustic image outside the head. More precisely, FIG. 2(a) explains how sound coming from a speaker is picked up, and FIG. 2(b) explains how a twin earphone or a stereophonic headphone catches sound. In FIG. 2(a), reference numeral 101 denotes a sound-source signal, reference numeral 103 designates a speaker, and reference numeral 102 denotes two microphones set in the outer-ear canals, respectively. In FIG. 2(b), reference numeral 104 designates an earphone or a headphone, reference numeral 105 denotes a digital filter. Note that suffix L in HRTFL and suffix R in HRTFR stand for “left” and “right” respectively.
The principal of finding the position of the acoustic image outside the head lies in electrically formulate a transfer function identical to the transfer function for sound traveling to the listener's eardrum from a sound source that exists outside the listener's head.
However, it is difficult for an electric signal emanating from a living body to pick up the vibration the eardrum are undergoing as sound waves. Hence, the transfer function of the electric signal traveling to the eardrum can hardly be measured accurately from the sound-source signal 101 shown in FIG. 2(a). This is why the listener sets small microphones 102 in his or her outer-ear canals, respectively, and the transfer function of the electric signal, i.e., head related transfer functions (HRTFs) in the left and right ears, is measured from the sound-source signal 101 that has been input to the speaker 103 by using these microphones 102.
The speaker 103 has a specific frequency characteristic. The true transfer function of the electric signal traveling from the input of the speaker 103 to the microphones 102 is therefore given as HRTF/SPTF, where SPTF is the transfer function for the speaker 103.
In the system of FIG. 2(b) of Publication 1, the twin earphone or stereophonic headphone 104 may be used to provide a transfer function that is equivalent to function HRTF/SPTF. To provide this transfer function, the transfer function of a signal traveling from the earphone or headphone 104 to the microphones 102 set in the outer-ear canals, i.e., ear-canal transfer function (ECTF), is measured. If the product of this transfer function ECTF and the transfer function of the digital filter 105 is equal to the transfer function HRTF/SPTF, aural signal identical to the speaker signals can be reproduced at the microphones 102 set in the outer-ear canals.
In the system disclosed in Publication 1, an ex-head sound-image locating means of the type shown in FIG. 5 is used to measure the outer-ear canal transfer function, i.e., transfer function attained while the listener is wearing the earphone or headphone 104. The outer-ear canal transfer function thus measured is corrected by using an adaptive equalization filter.
Microphones 3 that pick up the sound in the outer-ear canals are formed integral with the speakers of the earphone or headphone, as is illustrated in FIG. 1 of Publication 1. A digital filter 11 is used, which stores an impulse response having transfer function HRTF/SPTF that has been measured by such a configuration as shown in FIG. 2(a) of Publication 1.
A band-pass filter 13 is provided, for the following reason. An adaptive filter 12 and the transfer function ECTF are connected in series, and the output of this series circuit may be an impulse. In this case, the transfer function of the adaptive filter 12 is inverse to the function ECTF, i.e., 1/ECTF. However, the function ECTF pertains to both a speaker 1 and the microphones 3 and therefore attenuates outside a specific band. Hence, the transfer function of the adaptive digital filter 12, which is inverse to the transfer function ECTF, attains a large gain outside the specific band.
The tap coefficient or impulse response of the adaptive digital filter 12 can therefore be stably acquired if the result of the convolution performed on the impulse responses of the filter 12 and ECTF is regarded as the impulse response of the band-pass filter 13. In other words, if the band of the band-pass filter 13 is narrower than that of the adaptive digital filter 12, a subtracter 14 will cancel the ex-band part of the transfer function of the adaptive digital filter 12. As a result, a stable solution can be obtained.
In the system disclosed in Publication 1, an adaptive equalization filter is used to correct the outer-ear canal transfer function. In order to correct this transfer function accurately, the microphones 3 must exhibit flat frequency characteristic within the band. This is because the music will sound strange at the eardrum if the adaptive digital filter 12 generates an inverse transfer function from the transfer function ECTF that pertains to the characteristic of the microphones 3. Further, the position of the microphones 3 is important and should therefore be carefully determined. If the microphones 3 are located at the eardrums, no problems will arise. If the microphones 3 are located at the distal ends of the twin earphone or headphone (not at the ends of the outer-ear canals), however, it will pick up sound not at the nodes of a standing sound wave. Consequently, the microphones 3 will acquire such a characteristic that they catch sound at the dips of the standing sound wave. The music will inevitably sound strange to the listener.
Jpn. Pat. Appln. KOKAI Publication No. 2002-209300 (hereinafter referred to as Publication 2) discloses a technique of cancelling the influence of standing waves formed in a twin earphone or headphone and at the listener's eardrum. To cancel the standing waves, the vibration signal emanating from either eardrum should be measured to determine the sound-transfer characteristic in the outer-ear canals. It is difficult, however, to set microphones at the eardrums to detect the vibration signals in the vicinity of the eardrums. In the technique disclosed in Publication 2, the microphones are set at the eardrums of a pseudo-head, in order to measure the outer-ear ear canal transfer function. Based on the characteristic measured, a filter is designed, which can cancel the standing wave that extends from either eardrum and the earphone or headphone.
However, the length and acoustic impedance of outer-ear canals differ, from person to person. The transfer function in the outer ears therefore differs, on the individual basis. It follows that the position where resonance frequency is attained differs, on individual basis, too. Further, the resonance frequency is attained at a position in the left ear, and at a different position in the right ear. The outer-ear canal transfer function should therefore be corrected in accordance with the physical characteristics of the ears of each person. Hence, the characteristic determined by using the pseudo-head can hardly serve to manufacture a filter that proves satisfactory to all users. In view of this, filters of different characteristics may be prepared so that the user may select one that he or she finds best. Here arises a problem. The user can hardly select a filter he or she thinks the best for him or her. Moreover, the filter the user selects can scarcely work flawlessly.
Jpn. Pat. Appln. KOKAI Publication No. 9-187093 (hereinafter referred to as Publication 3) discloses a system that has an electro-acoustic converting means and a resonance-frequency component reducing means connected to the input of the electro-acoustic converting means. The resonance-frequency component reducing means is configured to reduce a resonance-frequency component of a frequency near the resonance frequency in human ears. Thus, the means prevents a decline in the hearing ability of the user who habitually listens to laud music through an earphone or a headphone. That is, the resonance-frequency component reducing means prevents the sound level of the resonance frequency in the ears from increasing excessively. The resonance-frequency component reducing means is an electrical circuit that has a resister, to which a parameter for reducing the resonance-frequency component detected is set. However, no parameters are specified in Publication 3. Methods of determining such a parameter are known in the art. One method is to use a filter inverse to the resonance data actually acquired as described in Publication 1. Another method is to provide a filter similar to the data acquired by, for example, a parametric equalizer. These methods are, however, disadvantageous in the following respects.
1) Since microphones cannot be located at the eardrums, the characteristics of the ears cannot be accurately measured. If the inverse filter designed on the basis of the characteristics measured is subjected to convolution, the resulting sound will be degraded in quality.
2) Many parameters are applied, rendering the tuning extremely difficult. Desirable characteristics may not be attained in some cases. Even if desirable characteristics are attained, it will be very difficult to determine the phase accurately.
As has been described, the conventional apparatus for rectifying resonance in the outer-ear canals cannot easily rectify the resonance in accordance with the structure of the outer-ear canals of each person.